The present invention relates to a process for the production of isocyanates in which the vapor mixture formed when phosgene and amine are reacted is subjected to at least one isothermal absorption treatment and at least one adiabatic absorption treatment.
The production of isocyanates is well known in the prior art. As a rule, phosgene is used in a stoichiometric excess with respect to the amine or a mixture of two or more amines. Processes for the production of organic isocyanates from primary amines and phosgene are described in the literature, for example in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Vol. A 19 p. 390 ff., VCH Verlagsgesellschaft mbH, Weinheim, 1991 and G. Oertel (Ed.) Polyurethane Handbook, 2nd Edition, Hanser Verlag, Munich, 1993, p. 60 ff. as well as G. Wegener et al. Applied Catalysis A: General 221 (2001), p. 303-335, Elsevier Science B.V.
The synthesis of the phosgene used in the amine phosgenation is well known and is described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 3rd Edition, Vol. 13, pp. 494-500. Other processes for the production of phosgene are described, for example, in U.S. Pat. No. 4,764,308 and WO 03/072237. On an industrial scale, phosgene is mainly produced by reacting carbon monoxide with chlorine, preferably on activated charcoal as catalyst. The highly exothermic gaseous phase reaction is carried out at temperatures of at least 250° C. to at most 600° C., as a rule in shell-and-tube reactors. The heat of reaction may be dissipated in various ways, for example, by means of a liquid heat exchange medium, as described, for example, in WO 03/072237, or by hot cooling via a secondary cooling circuit with simultaneous utilization of the heat of reaction in order to generate steam, as disclosed, for example, in U.S. Pat. No. 4,764,308.
In the amine phosgenation, unreacted phosgene mostly occurs at least in part in gaseous form together with the released hydrogen chloride. Phosgene and hydrogen chloride fractions still contained in the liquid isocyanate-carrying product stream are separated during the course of the isocyanate working-up stage. As a rule, this product stream can still contain fractions of solvent, inert gases, such as for example nitrogen and carbon monoxide, and secondary products of the phosgene synthesis, such as carbon dioxide and possibly entrained isocyanate. In order to operate the process for the production of isocyanates as economically as possible, it is essential to recover the excess phosgene with losses as small as possible and recycle it to the phosgenation process. It is also necessary to separate the stoichiometrically occurring hydrogen chloride gas and use it. Suitable uses for the hydrogen chloride are determined on the basis of the purity requirements on the hydrogen chloride for any given application.
Possible uses for hydrogen chloride include marketing the aqueous solution (hydrochloric acid) or using hydrochloric acid in other industrial or chemical processes. One of the most common possible ways of using gaseous hydrogen chloride is for the oxychlorination of ethylene with hydrogen chloride to form ethylene dichloride. Recycling processes for the hydrogen chloride and the return of the chlorine and/or hydrogen to the production process in which the hydrogen chloride is formed are also among the preferred procedures. These recycling processes include the catalytic oxidation of hydrogen chloride, for example according to the Deacon process, the electrolysis of gaseous hydrogen chloride, as well as the electrolysis of an aqueous solution of hydrogen chloride (hydrochloric acid). WO-A-04/14845 discloses a process for the catalytic oxidation according to the Deacon process, and WO-A-97/24320 discloses a process for the gas phase electrolysis of hydrogen chloride. A review of electrochemical recycling processes is given in the article “Chlorine Regeneration from Anhydrous Hydrogen” by Denne Turin Mah, published in “12th International Forum Electrolysis in Chemical Industry—Clean and Efficient Processing Electrochemical Technology for Synthesis, Separation, Recycle and Environmental Improvement”, Oct. 11-15, 1998, Sheraton Sand Key, Clearwater Beach, Fla.
The electrochemical oxidation of an aqueous solution of hydrogen chloride (hydrochloric acid) using a gas diffusion electrode as cathode is described in WO-A-00/73538 and WO-A-02/18675.
In the electrolysis of aqueous hydrogen chloride by the diaphragm or membrane process, hydrochloric acid is used as electrolyte in the anode space as well as in the cathode space. In the electrolysis, chlorine is produced at the anode and hydrogen is produced at the cathode.
The aforementioned possibilities of using hydrogen chloride impose certain purity requirements and thus involve expenditure and equipment on purification after separating the majority of the other components in the gaseous stream containing phosgene and hydrogen chloride. The catalytic hydrogen chloride oxidation by the Deacon process is carried out with a catalyst that requires the preliminary purification of the hydrogen chloride gas from a phosgenation process by means of absorption on a purification bed, or the catalytic combustion of solvent residues contained in the hydrogen chloride (WO-A-04/014845). In the gas phase electrolysis of hydrogen chloride by the so-called solid electrolyte systems according to WO-A-97/24320, a contamination of the ion exchange membrane or of the catalytically active material is unallowable in order to avoid having to replace the units. In the electrochemical oxidation of an aqueous solution of hydrogen chloride using a gas diffusion electrode as cathode, it is suggested in WO-A-02/18675 that the hydrogen chloride (hydrochloric acid) be purified by means of activated charcoal and, if necessary, additionally by means of an ion exchange resin. For the use of hydrogen chloride gas in oxychlorination, a two-stage condensation may be used to separate interfering impurities, such as solvent residues (U.S. Pat. No. 6,719,957).
An aqueous solution of hydrogen chloride (hydrochloric acid) for use in the foodstuffs industry must have a correspondingly high degree of purity, which can be achieved by an adsorptive post-purification on a bed of activated charcoal, as known from the prior art.
The treatment of the phosgene-containing and hydrogen chloride-containing substance streams from the isocyanate production according to the prior art is described hereinafter.
The general aim is to isolate the substance streams containing phosgene and hydrogen chloride with secondary components contained therein, such as solvent, in the required purity as economically as possible in order to be able to reuse phosgene in the amine phosgenation and pass the hydrogen chloride to a suitable utilization stage. The processes of condensation, partial condensation, washing/scrubbing, absorption, adsorption and distillation are normally employed for this purpose.
A partial condensation of phosgene from the process gas may be achieved under high pressure, for example at a pressure between 10 and 50 bar, in an energy-efficient manner by means of cooling water, though for exploitation on an industrial scale the stringent safety measures as regards a leakage involving escape of phosgene have to be taken into account, as described in DE-A-3212510.
The phosgenation reaction and working-up of the gas phase under elevated pressure is also described in U.S. Pat. No. 3,544,611. In a pressure range of from 10 to 50 bar, the process gas is cooled with water in order to condense a large part of the phosgene that is used in stoichiometric excess. A further depletion of phosgene in the hydrogen chloride stream requires the use of cooling agents. In this case, the economic advantage of the amine phosgenation with working-up of the process gas at elevated pressure is also reflected in the saving of energy for cooling the phosgene condensation. An alternative is described in U.S. Pat. No. 3,544,611 in which hydrogen chloride is condensed from the process gas stream at 33 bar and at a coolant temperature of −20° C. In this case, phosgene is condensed by cooling with water and separating in a preliminary stage. The respective purities required of the two components is achieved by a distillation/stripping column between the two condensation stages.
In DE-A-10260084 reference is made to U.S. Pat. No. 3,544,611 in connection with an increased potential hazard in the event of a leakage due to this pressurized procedure. It is also noted that in the described processes there is an undesirably high concentration of hydrogen chloride in the phosgene used for the phosgenation, and phosgene is also lost with the hydrogen chloride stream (first variant). In the second variant, apart from the comments on the already-mentioned potential hazard, reference is also made to the energetically unfavorable hydrogen chloride liquefaction at low temperatures and high pressures. For further utilization, the hydrogen chloride must then be evaporated, which again uses energy.
In the process disclosed in GB-A-827376, an amine phosgenation is carried out at a pressure of ca. 3 bar. After completion of the reaction, excess phosgene and hydrogen chloride that are formed are separated overhead in a column at elevated temperature. Phosgene is condensed out from the gaseous phase, and the hydrogen chloride is flashed (expanded) and removed. However, high residual amounts of phosgene in the hydrogen chloride as well as undesirably high hydrogen chloride contents in the recovered phosgene can be expected with such a simple separation.
Amine phosgenation in chlorobenzene to form TDI and MDI is described in U.S. Pat. No. 3,3812,025. After completion of the reaction, solvent together with phosgene and hydrogen chloride are distilled off, chlorobenzene and phosgene are then condensed and recycled to the phosgenation, and hydrogen chloride containing considerable residual amounts of phosgene is passed through an absorber to eliminate the phosgene. In this case too the phosgene/hydrogen chloride separation is incomplete in both streams, so that phosgene losses occur through hydrogen chloride and undesirably high hydrogen chloride fractions are contained in the phosgene, which promote a disadvantageous amine hydrochloride formation in the phosgenation.
The amine phosgenation to isocyanate combined with working-up disclosed in SU-A-1811161 is described in DE-A-10260084. In DE-A-10260084, it is also reported that phosgene is absorbed in gaseous form in the solvent chlorobenzene without prior condensation. After the phosgenation reaction, hydrogen chloride, phosgene and to some extent solvent are separated as gaseous phase. After partial condensation, the gaseous phase is passed to an absorber, the liquid phase is led into a stripping column in which hydrogen chloride and phosgene are separated overhead, and is partially condensed and likewise passed to the absorber. A solution of ca. 70 wt. % of phosgene in chlorobenzene is formed in the absorber. The gaseous hydrogen chloride stream from the absorber head still contains ca. 4% of phosgene and is passed to a further treatment stage. According to DE-A-10260084, the solution of phosgene in chlorobenzene also still contains relatively large amounts of hydrogen chloride due to the chlorobenzene wash at low temperature. According to the details given in DE-A-10260084, hydrogen chloride and phosgene after their separation are still mutually contaminated to such an extent that, as already described, hydrogen chloride cannot be passed without further working-up to one of the conventional utilization stages and the phosgene solution that is obtained is uneconomical for the phosgenation process.
In EP-A-0570799, which is a publication relating to amine phosgenation in the gaseous phase, reference is made to the separation in a manner known per se of excess phosgene after condensation of the isocyanate that is formed. This may be achieved by means of a cold trap, absorption in an inert solvent (e.g., chlorobenzene or dichlorobenzene) maintained at a temperature of −10° C. to 8° C., or by adsorption and hydrolysis on activated charcoal. The last variant does not appear economically feasible for large-scale implementation. The hydrogen chloride gas passing through the phosgene recovery stage can be recycled in a manner known per se to recover the chlorine required for the phosgene synthesis.
A continuous two-stage amine phosgenation process in the liquid phase is described in U.S. Pat. No. 3,226,410. A phosgene solution is admixed in a stoichiometric excess of an amine solution in a tubular reactor at temperatures of up to 90° C. The second stage takes place in a boiler at 110° to 135° C. The gaseous phase, composed of phosgene, hydrogen chloride and solvent fractions, is removed overhead from the second stage, condensed in a two-stage process, and passed to the phosgene solution vessel. Non-condensable fractions pass into an absorption column, where phosgene still contained in the gas stream is absorbed by means of distilled-off solvent from the liquid phase of the phosgenation and is passed to the phosgene solution vessel. Non-absorbed fractions from the absorption column which for the most part are hydrogen chloride gas, are fed to an HCl absorber operated with water, in which aqueous hydrochloric acid is formed.
Apart from the isocyanate production, a phosgene/hydrogen chloride separation is also necessary in the phosgenation of alcohols to form chloroformates. In accordance with the process disclosed in DE-A-69820078, this takes place at high pressures in a column connected downstream of the reactor. The reactor pressure ranges from 2 to 60 bar, preferably 6 to 40 bar. When using high pressure in the phosgene/hydrogen chloride separation, it is pointed out that, from the economic aspect, the condensers no longer have to be operated at low temperatures. In DE-A-3000524 and U.S. Pat. No. 3,211,776, reference is simply made to a blowing-off of excess phosgene in the working-up after the chloroformate formation from alcohol, phosgene and catalyst.
A chemical separation of hydrogen chloride and phosgene is less significant for the industrial production of isocyanate because of the extensive use of, for example, bases, loss of hydrogen chloride, and high incidence of by-products. For example, in EP-A-1020435 and DE-A-1233854, tertiary amines are used as hydrogen chloride traps which precipitate out as solids in the form of the hydrochloride. Alkali metal or alkaline earth metal salts or oxides are used for this purpose in JP-A-09208589.
The aim of obtaining the purest possible hydrogen chloride and pure phosgene from a substance mixture such as is normally used in the production of isocyanates by reacting amines with phosgene is adopted in DE-A-10260084. A four-stage process is described, the essential stages of which require two separate columns and additional equipment. The process gas from the isocyanate production is composed mainly of phosgene, hydrogen chloride, solvent fractions, low-boiling compounds and inert substances (e.g., carbon monoxide and carbon dioxide). The first process step is the partial condensation of the process gas, which can take place in one or more stages, wherein depending on the equipment pressure, the procedure may be operated between 40° C. by means of cooling water and −40° C. with brine cooling. The partially condensed mixture thereby obtained is then passed, between the stripping section and the rectifying section, to the following distillation column. In the given example, using chlorobenzene as solvent, this column is designed as a bubble-cap column with 22 trays in the stripping section and 11 trays in the rectifying section. The column serves to remove hydrogen chloride from the phosgene and is equipped for this purpose with a forced circulation evaporator (Robert evaporator) and a shell-and-tube heat exchanger as head condenser. At 24.5° C. feed temperature, 38° C. bottom temperature, −9° C. head temperature and 2.5 bar head pressure, the reflux temperature of the partial condensate at the column head is −20° C. Under these conditions, the bottom product has a hydrogen chloride content of 0.01 wt. %, phosgene content of 89 wt. % and chlorine content of 10 wt. %. This stream is passed to the reaction part of the isocyanate synthesis.
As an alternative to the aforementioned evaporator in the distillation column, the removal of hydrogen chloride from the process waste gas stream to be treated may also take place by stripping from the process waste gas stream to be treated with an inert gas such as nitrogen, with the process solvent vapor, phosgene, or with another gaseous substance or substance to be evaporated.
The non-condensed fraction containing 74 wt. % of hydrogen chloride and 26 wt. % of phosgene in the head condenser of the distillation column is led at −20° C. into the lower region of an absorption column that is equipped with three sections of wire-gauze rings. Chlorobenzene at a temperature of −25° C. is added to the head of the washer, and the heat of solution of hydrogen chloride in the chlorobenzene is removed with an intermediate cooler operating at −30° C. Vapors are formed at the head of the washer, which are fed after a demister to a head condenser operating at −30° C. Here droplets are retained, which together with a small fraction of condensed vapors are returned to the bottom of this absorber or washer. The head of the column is operated at 2.2 bar and at −8° C., and the bottom at 6° C. The product removed from the head after the condenser has a hydrogen chloride content of 99.5 wt. %, a phosgene content of 0.1 wt. % and a chlorobenzene content of 0.1 wt. %. The product removed from the bottom contains 19 wt. % of phosgene, 78 wt. % of chlorobenzene and 3 wt. % of hydrogen chloride.
In the example of DE-A-10260084, the gaseous overhead extract is subsequently purified with an activated charcoal filter. The phosgene or chlorobenzene residues could not be detected by gas chromatography analysis or by IR spectroscopy.
The bottom extract from the absorber containing the aforementioned contents of phosgene and hydrogen chloride should then be passed as reflux liquid to a reaction column, to a column for the phosgene separation, or for working-up the reaction mixture. In the last case, reference is made to the possibility of saving a vapour condenser for generating the reflux liquid.